1. Field of the Invention
This invention relates in general to means and methods for stabilizing the properties of polycrystalline semiconductor layers and, more particularly, to means and method for controlling surface hillock growth in polycrystalline semiconductor layers during high temperature processing.
2. Background Art
Electronic devices using polycrystalline semiconductors often exhibit problems associated with the formation of surface roughness in the poly. For example, when un-capped layers of polysilicon are heated, such as during activation or redistribution of a dopant, it is observed that bumps, hillocks, and/or spikes form on the surface of the poly. These uneven surface features, collectively referred to herein as hillocks, can degrade the integrity of any subsequent layers placed on top of the poly, as for example dielectrics used for passivation or multi-layer interconnections.
It is known that hillock formation may be avoided in polysilicon, for example, by capping the poly layer prior to heating. Capping consists of applying a covering layer of another material such as silicon dioxide or silicon nitride. However, under certain circumstances it is not practical to use a cap layer over the poly, and in these situations hillock formation has heretofore been unavoidable. Examples of such situations are when a high temperature processing step must be used prior to providing the cap layer, or when the insulating cap layer would interfere with making good electrical contact to the poly layer, or where the cap layer would interfere with subsequent processing steps. Thus, a need continues to exist for improved means and methods for overcoming hillock formation and other problems arising in connection with polycrystalline semiconductor layers.
Accordingly, it is an objective of the present invention to provide improved means and methods for preventing hillock formation in the surface of polycrystalline semiconductor layers during heat treatment.
It is an additional object of the present invention to prevent hillock formation in polycrystalline semiconductor layers without using a cap layer.
It is a further object of the present invention to prevent hillock formation in polycrystalline semiconductor layers without using a surface insulating layer.
As used herein the words "dopant" or "doping" are intended to refer to the class of impurities which provide substantial number of shallow donor or acceptor levels in the semiconductor so as to substantially alter the conductivity of the material. In the case of silicon materials, examples of this class of dopants are arsenic, boron, phosphorous, antimony, aluminum, gallium, and indium, but not oxygen or nitrogen. Those of skill in the art will understand that other such dopant elements exist for use in silicon and other semiconductor materials. As used herein the words "high temperature" are intended to refer to temperatures where the phenomena of interest are proceeding at a rate significant compared to the time periods of interest. For example, for dopant migration or oxidation of silicon, this is generally above about 1000 degrees C. Those of skill in the art will understand that different temperatures apply for different phenomena and materials.